net/mlx4_core: Defer VF initialization till PF is fully initialized

Fix in commit [1] is not sufficient since a deferred VF initialization
could happen after pci_enable_sriov() is finished, but before the PF is
fully initialized.
Need to prevent VFs from initializing till the PF is fully ready and
comm channel is operational.

[1] - 97989356

 "net/mlx4_core: mlx4_init_slave() shouldn't access comm
      channel before PF is ready"

CC: Stuart Hayes <Stuart_Hayes@Dell.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>